R. S. Tarr — Superimposition of Drainage in Texas. 359 



Aet. XLY. — Superimposition of the Drainage in Central 

 Texas; by Ealph S. Tarr. 



In the number of this Journal for last May the writer de- 

 scribed the general history of the drainage system of Central 

 Texas as far as it could be read with our present knowledge of 

 the region. In that paper it was shown that the present drain- 

 age is superimposed, having originated upon the Cretaceous 

 strata in Tertiary times and, after removing this covering from 

 the buried Paleozoic rocks, finding itself superimposed upon 

 these hard rocks. There is abundant proof of this ; for the 

 central Paleozoic region is only partially uncovered and the 

 denudation of the Cretaceous is still in progress. One of the 

 chief effects of the superimposition is that the Colorado river, 

 one of the great arteries of eastern Texas, flows, in its middle 

 course, for many miles over hard Silurian marble containing 

 great quantities of flint. This barrier, accidentally discovered 

 in cutting through the Cretaceous, has retarded the river in its 

 down cutting. 



Since writing the former paper two points connected with 

 the drainage of Central Texas, which then puzzled me, have 

 become clear ; and as they are interesting confirmations of the 

 superimposition theorj^ for the origin of the present drainage 

 of the region I give them below. 



A superimposed river, having selected its course with refer- 

 ence to a structure now no longer present, naturally finds itself 

 flowing without reference to the nature of the newly discov- 

 ered beds. Thus it is that the Colorado in central Texas is 

 now busy with a barrier of hard Silurian rock, and thus it is 

 that this river flows with a general course at right angles to 

 the strike of the Carboniferous rocks and in an opposite direc- 

 tion to the dip. 



Not only the Colorado itself, but all its tributaries flow with- 

 out especial reference to structural weakness ; but the smaller 

 branches take advantage of the structural peculiarities, show- 

 ing, in many cases, that they are more recent in origin than the 

 time of removal of the Cretaceous. Moreover, some of the 

 medium-sized streams, which in their upper and middle course 

 flow, perhaps on Cretaceous, quite regardless of Carboniferous 

 structure, have nearer their mouth partly readjusted themselves 

 to the new conditions. The number of strike valleys in the 

 lower course of such streams is quite astonishing since it shows 

 to how great an extent drainage is dependent upon structure 

 and how readily, even under great disadvantages, streams will 

 make use of such weaknesses. In the Carboniferous this is 



Am. Jour. Sci.— Third Series, Vol. XL, No. 239. — Nov., 1890. 

 23 



